1. Field of the Invention
The present invention relates, in general, to a lighting unit having an incandescent bulb for an outdoor visual display sign, and, more particularly, to a lens for the lighting unit that provides a graduated horizontal distribution of the emitted light.
2. Statement of the Problem
People have long used outdoor signs for advertising, sending messages to others, and displaying artwork. In early times, carved and painted wooden signs were hung outside of taverns and inns to advertise their presence and accessibility for travelers. In more recent times, large billboards are erected near highways and messages are painted on the sides of barns or other large buildings to capture the attention of passing motorists. Such large billboards present an advertisement or message in a static form. That is, the billboard is most often painted or has a previously prepared advertisement applied to it. The message remains on the sign until it is desired to replace the message with a different communication. These billboards are often illuminated during the hours of darkness in order to reach even more viewers. Such illumination is usually achieved with incandescent or other bright lights.
A need exists for large outdoor visual display signs that are capable of vivid color representation and have the ability to display moving images and that can be placed at remote locations such as on a building or a freestanding pylon.
Although visual display systems exist providing moving images in color, for example, a television show or videotape, such display signs have not provided vivid video images, especially signs that are used outdoors and especially during bright sunlight. Previous signs designed for outdoor use have been prohibitively expensive to operate, to purchase, and to maintain. Lack of brightness is a serious problem for prior outdoor display signs. Light from other sources, especially the sun and the moon, but also including, but not limited to, other lights on the ground or on adjacent buildings, reduces the ability of viewers to perceive the picture displayed by the sign. These outdoor display signs are comprised of a plurality of lighting units, some of which are turned off, and therefore black, at any particular moment, and some of which are lit in order to show the picture. Ambient light reduces the contrast of the sign in two ways. First, when a lighting unit is off, the blackness of the lighting unit is only as dark as the ambient light. Second, when a lighting unit is lit, its colors mix with the ambient light. Thus, external light washes out the picture, causing the viewer to have difficulty perceiving the picture being displayed, especially when that picture is a moving picture in color. In addition, visual display signs are often placed in high locations so that they can be seen from a distance, and the main light output is directed horizontally outward. As a result, viewers who are not directly in front of the sign, but are viewing the sign from the side as they pass by on foot or in automobiles, do not receive the most direct light output. Thus, a need exists for a lens that will increase the light output of a visual display sign in a particular desired direction.
Various methods are used to increase display sign brightness as perceived by a viewer who is not directly in the path of the light output. For example, a high-intensity light source or light bulb can be used. This solution is expensive. The lighting units can be placed at a predetermined angle to direct light output in a desired direction, such as downward toward a viewer below the sign, or a mirrored shade can be placed over the lighting unit and angled downward. None of these methods improves the delivery of light output to viewers who are not directly in front of the display sign. Thus, a need exists for a lens having a predetermined shape for the lighting units of large visual display signs. The lens should disperse the light emitted from each lighting unit of the display sign more horizontally than vertically, concentrating the emitted light in a narrow horizontal band so that, as a viewer moves away from the front of the display sign toward the side of the display sign, the displayed image remains bright and legible. The horizontal spread also avoids loss of light uselessly in the upward direction, concentrating the emitted light where it is best seen by a viewer below and/or to the side of the sign.
Numerous examples exist in the prior art of lenses having a predetermined shape in order to direct emitted light in a desirable direction or pattern. A patentability search for lenses having a predetermined shape in which the emitted light is spread further horizontally than vertically resulted in the following patents.
U.S. Pat. No. 2,853,599 discloses a lens formed from a single piece of cast glass that produces a beam of light having a horizontal width wider than its vertical height. The light-incident face of the lens has concave arcuate ridges that spread the emitted light horizontally, whereas the light-emergent face of the lens has concentric annular ridges of convergent refracting power. The ridges on the light-emergent face of the lens act to distribute the emitted light uniformly.
U.S. Pat. No. 1,671,313 discloses a lens for headlights in which the body of the lens is generally prismoidal with the base of the prism at the bottom. The light-incident and light-emitting faces of the lens are grooved to form a series of minus cylindrical lenses extending vertically. These grooves diffuse the light and spread the light horizontally.
U.S. Pat. No. 1,955,597 teaches a modified Fresnel lens that has a spherical front face and a rear face on which is formed a series of substantially semicircular refracting zones and a central bulls eye. The refracting zones on the upper half of the rear face have a different configuration from the refracting zones on the lower half of the rear face, so that the light emitted by the upper half of the lens is largely parallel and the light emitted by the lower half of the lens is spread away from the central axis.
None of the above-described patents provides a simple, inexpensive lens for spreading light evenly and horizontally so that a viewer of a visual display sign comprising the lenses is provided with bright, even illumination whether the viewer is directly in front of or is below and/or to the side of a lighting unit emitting light through the lens.
3. Solution to the Problem
Under the teachings of the present invention, a lens is disclosed that, in the first embodiment, has on its light-incident surface a plurality of vertically disposed ridges derived from a predetermined high-frequency waveform, whereas the light-emitting surface of the lens is a substantially flat surface. The high-frequency waveform is derived from a combination of a first predetermined primary waveform and the third harmonic waveform of the first primary waveform. The lens, when attached to a lighting unit for a visual display sign, spreads the emitted light about twenty degrees horizontally as measured from the planar circumference of the lens, enabling the viewer of a visual display sign comprising the lenses to perceive a bright video image whether the viewer is directly in front of or is to the side of the visual display sign, or whether ambient conditions are bright, such as outdoors in sunlight.
In a second embodiment of the lens of the present invention, the high-frequency waveform of the light-incident surface is derived from a combination of a second predetermined primary waveform and the third and fifth harmonic waveforms of the second primary waveform. The light-emitting surface is formed into a high-frequency, low-amplitude waveform, rather than being flat as in the first embodiment. This second embodiment of the lens, when attached to a lighting unit for a visual display sign, spreads the emitted light forty-five degrees horizontally as measured from the planar circumference of the lens. Both embodiments of the lenses made according to the present invention spread light emitted from the lens with graduated intensity horizontally, without any hot spots or darker areas.